This invention relates to a sensing device to monitor the level of a liquid in a vessel and more particularly, to an apparatus and a method for detecting the level of a liquid utilizing a propagated carrier frequency.
As one can ascertain, the prior an is replete with many devices which operate to monitor the fluid level in a tank or other storage vessel. Many devices operate by means of an electromechanical switch which is activated by a float. The float rides or floats on the surface of the liquid, and when the liquid reeves to a given value, a switch coupled to the float is operated. These types of devices are subject to sticking in either the on or off position. Most of the devices are operated so that they control a pump or other means for controlling the level of fluid in a storage tank or vessel. Thus, if such a switch sticks in the closed position, it will cause the pump to continue to operate and create problems. On the other hand, if the switch locks in the open position this can cause flooding or overflow because the pump or other device will not operate. Such system failures are well known and occur in many instances.
There are other systems which utilize electrical or electronic devices to detect the level of fluid in a vessel or otherwise. Such devices are also well-known. In any event, the detection of fluid in such vessels is extremely important in many different applications. A particular application is to monitor the fluid level in a steam boiler. The function of such a device is to open a set of electrical contacts in the event of a low water condition in the boiler, thereby shutting down the burner and protecting the boiler against thermal damage or fire. Approximately two million residences in the United States are heated using low pressure steam. There is a significant demand for replacement boilers and associated controls and each of these boilers must be outfitted with a low water cutoff device to prevent the above-noted problems.
As indicated, many of the prior art devices use float-type controls. Electronic low-water cutoff devices have been available from many different manufacturers. As will be explained below, certain of these devices have serious problems in the reliable sensing of fluid levels.
It is of course understood that a major consideration in the design or operation of any such device is reliable, fail-safe operation at relatively high temperatures which device should also be easy to install while being economical and reliable.
It is an object of the present invention to provide a low-water cutoff device or fluid level detector employing a carrier frequency sensing technique.